Airbus does the math

MathWorks' senior engineer, Mark Walker explains how Airbus has developed a fuel management system for the A380 using the company's model-based design. Model-based design is a mathematical and visual approach to solving some of today's toughest design challenges. The approach is applied in a multitude of today's industries to model, simulate control logic, communicate functional specifications, and accelerate development of design.

In aerospace, modelling software has helped engineers take a systematic approach to dealing with the growing complexity of aircraft design while reducing time to market. The approach enables engineering teams to move from internal research and development to design and implementation in a single environment. The Airbus A380, the largest commercial aircraft currently in operation, has a range of more than 8,000 miles. To enable such long nonstop flights, the A380's 11 fuel tanks have a capacity of 250 metric tons (320,000 litres).

The A380's sophisticated fuel management system handles fuelling and de-fuelling operations on the ground, as well as fuel flow to engines and between tanks while airborne. The system can move fuel between tanks to optimise the aircraft's centre of gravity, reduce wing bending, and keep fuel within the acceptable temperature range.

Airbus' engineers used industry-standard tools Simulink and Stateflow to develop a model of the fuel management system that was reused throughout the project. With model-based design, Airbus was able to represent the functional specification to validate requirements months earlier than was previously possible.
“On earlier projects, it took up to nine months to integrate the fuel system design with the simulated cockpit, or Iron Bird,” states Airbus' computational analysis expert in fuel systems, Christopher Slack. “Using model-based design on the A380, it took less than a month. Similarly, by reusing the model to commission the hardware-in-the-loop (HIL) rig, we saved three months of development and shortened the time from initial concept to first flight.”

The design challenge

The A380's fuel management system had to be able to safely handle any failure in the system's 21 pumps, 43 valves, and other mechanical components. In a complex system, it's challenging for engineers at the requirements stage to predict problems that can result from combinations of relatively minor failures.
The fuel system specification document for the A380's predecessor, the A340, had over 1,000 written requirements. The fuel system of the A380 is three to four times more complex. Due to the additional complexity, text requirements can leave room for ambiguity and misinterpretation. With that many requirements, it's difficult for anyone to understand all the possible interactions and identify conflicts in a short time frame.

Airbus decided to use model-based design software to model the A380's fuel management system, validating requirements through simulation, and clearly communicating the functional specification. Simulink and Stateflow were used to model the system's control logic, which comprises 45 top-level charts, almost 6,000 states, and more than 8,700 transitions. This model defined modes of operation on the ground (including refuel, defuel, and ground transfer) and in flight (including normal engine feed, centre of gravity control, load alleviation, and fuel jettison).

The functionality within each top-level mode was grouped into sub-charts, enabling engineers to work independently on individual components in the hierarchy.

The team then developed a parameterised plant model of the tanks, pumps, valves, and electrical components using Simulink. Engineers could then set parameter values to configure this model to represent fuel systems for any Airbus aircraft.

After running closed-loop simulations of individual operational components in Simulink, the team integrated them into a complete model for system-level simulations.

Using Parallel Computing Toolbox and MATLAB Distributed Computing Server, the team then performed Monte Carlo simulations on a 50-worker computing cluster. Over a weekend, they could run 100,000 simulated flights under varied environmental conditions and aircraft operational scenarios.

The team created a desktop simulator by generating code from the plant and control logic models with Simulink Coder. The MATLAB based user interface then enabled suppliers, airline customers, maintenance engineers, and other Airbus teams to visualise how the fuel management system works and interacts with other aircraft systems.

The team also used the Simulink models to develop HIL tests and commission its HIL testing rig well before the real hardware was available.

After successful flight tests of the A380 software, the team was able to leverage MathWorks tools to tune its plant model using measured flight test data, remove noise from the test data, and evaluate differences between the measured data and predicted results in order to predict system performance beyond the usual flight envelopes.

Based on the success of implementing model-based design for the A380, Airbus engineers were able to handle a substantially more complex project with the same size engineering team and are now using this approach to develop the Airbus A350 XWB's fuel management system, reducing development time of this aircraft by one year.

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